1. Technical Field
The present invention relates to a system for transmission of a radio signal in a mobile communication network as well as to a corresponding method.
2. Description of Related Art
In traditional mobile communication networks a carrier signal being generated at base band level in a radio access node is amplified by a power amplifier and radiated over the air with linear polarisation, typically vertical or at +/−45° slant using a single Tx port.
The polarisation properties of the signal transmitted in the radio channel are modified by reflections from and refraction on multiples obstacles before arriving at the receiver of the mobile terminal, causing the power originally transmitted with a given linear polarisation to be progressively rotated and also to progressively decouple into the orthogonal polarisation (e.g. in to the horizontal if the transmitted wave was at vertical polarisation)
Most mobile terminals comprise a single antenna optimised for receiving a linearly polarized radio signal. In case of little decoupling and the antenna of the mobile terminal being aligned with the received polarization, the quality of the received signal is generally good. However, in case of misalignment at the mobile terminal and/or a strong decoupling between polarisations of the transmitted wave received by the mobile (as an outcome of the propagation of the radio mobile channel) and the polarisation of the mobile antenna system, the quality of the received signal may deteriorate significantly.
In order to improve the quality of the received signal, mobile terminals with receive diversity have been developed. Such mobile terminals comprise a cross polar antenna, for example consisting of two perpendicular dipole antennas, for receiving the radio signal through two independent channels with orthogonal linear polarisations. The signals of both reception branches are combined in the diversity receiver. Although the received signal quality of such mobile terminals is superior to that of the mobile terminals without diversity reception, the performance of Rx diversity implemented in such a way is not optimal under all circumstances. With such an approach the contributions from the two antennas of the mobile station is overall fairly unequal. Indeed the decoupling of the polarisation of the radio channel is often not strong enough to maximise the overall power received at both reception branches and therefore diversity is usually underperforming on average (long term fading), because the majority of the received signal is captured mainly through one of the reception branches in the antenna mobile station.
This effect is predominant under good and medium radio conditions, where multipath has not had a relevant effect on the decoupling between polarisations, yet. This results in the linear polarisation transmitted by the radio base to be still mostly present at the mobile terminal receiver, thus preventing any diversity gain to be achieved at all.